It's not as pretty as a gear motor, perhaps, but I've been really happy with my Harbor Freight 1/2" low-speed, high torque drill. It has a trigger lock, variable speed from 0-500 rpm that you can set, and is reversible.
It doesn't flinch starting with a full 10 lbs. in the hopper. To mount it, I re-purposed an old chair to use as stand, mounting the drill on its side at the same height as the mill base on top of a bucket, and then used a bolt through the bottom of the chair seat that threads into port where the side handle would mount to the drill. It's hands free operation for milling, plus I have a heavy duty drill to use to mix mortar or whatever, too. So it offers some benefits over a gear motor to me. They go on sale for $44 every so often (currently,through April) and you can use the 20% coupon in-store coupon on top of that.